Today's high speed communication systems require accurate knowledge of the group delay of a device as a function of wavelength. Many optical devices have group delays that depend on the input polarization state. Effectively, these optical devices have a polarization dependent group delay. Measurements performed with a polarimeter only enable the determination of the difference between the group delay of the fast polarization input state and the slow polarization input state. This limitation reflects the polarimeter's inability to measure the individual group delay of the fast polarization input state and of the slow polarization input state.
Typical polarimeters are capable of determining the Jones matrix of an optical device. The information contained in the Jones matrix is sufficient to determine many transmission properties of a two-port optical device, such as the differential group delay (DGD), polarization mode dispersion (PMD) and the polarization dependent loss (PDL). Unfortunately, the information provided by the Jones matrix is not sufficient to determine chromatic dispersion (CD) and group delay (GD) of the principal states of polarization. Chromatic dispersion is a variation in the speed of propagation of the lightwave signal with wavelength and group delay is the difference in transit times at different wavelengths. However, a polarization analyzer can only measure the relative group delay, i.e. DGD, between the two principle states of polarization.
Simova et al. in “Characterization of Chromatic Dispersion and Polarization Sensitivity in Fiber Gratings”, in IEEE Transactions on Instrumentation and Measurement, Vol. 48, No. 5, 939–943, 1999 disclose a measurement procedure in which a polarization analyzer is used in conjunction with a chromatic dispersion analyzer based on rf-modulation phase shift technique. In this measurement procedure, the two instruments are used together to determine DGD, PMD, PDL, GD and CD. This measurement procedure requires two relatively expensive and complicated instruments to be used independently of each other for measurements on the device under test (DUT). The DUT must be measured at separate times by two completely distinct measurement systems which may also introduce errors into the measurement process.
A precision of 10 femtoseconds has been demonstrated for polarimetric measurements for determining the differential group delay (DGD) (see Heffner, IEEE Photonics Technology Letters, vol. 4, no. 9, 1992) while interferometric measurements for determining the differential group delay (DGD) are typically an order of magnitude less precise.
Hence, a desirable measurement system for optical communication components needs to be able to quickly, easily and accurately measure desired optical parameters such as: DGD, PMD, PDL, GD and CD.